Substantial changes have occurred in the Arctic Ocean in the last decades. Not only sea ice has retreated significantly, but also the ocean at middepth showed a warming tendency. By using simulations ...we identified a mechanism that intensifies the upward trend in ocean heat supply to the Arctic Ocean through Fram Strait. The reduction in sea ice export through Fram Strait induced by Arctic sea ice decline increases the salinity in the Greenland Sea, which lowers the sea surface height and strengthens the cyclonic gyre circulation in the Nordic Seas. The Atlantic Water volume transport to the Nordic Seas and Arctic Ocean is consequently strengthened. This enhances the warming trend of the Arctic Atlantic Water layer, potentially contributing to the Arctic “Atlantification.” Our study suggests that the Nordic Seas can play the role of a switchyard to influence the heat budget of the Arctic Ocean.
Plain Language Summary
The Arctic sea ice decline is among the key indications of climate change, which has strong impacts on the environment, human beings, and biodiversity. In this paper we found that the Arctic sea ice decline at the surface can even cause Arctic Ocean warming at middepth by intensifying the upward trend of ocean heat supply to the Arctic Ocean through Fram Strait. The Nordic Seas play the role of a switchyard for the involved processes: The sea ice decline reduces the sea ice export through Fram Strait, which further increases the salinity in the Greenland Sea. Consequently, in the Nordic Seas the sea surface height decreases and the gyre circulation strengthens. These changes then increase the Atlantic Water inflow to the Nordic Seas and the Arctic Ocean, causing significant warming in the Atlantic Water layer of the Arctic Ocean. The changes in the ocean heat budget have strong implications on potential feedbacks to sea ice decline through basal melting in a future warming climate. The intensification of the Atlantic Water volume transport through Fram Strait can impact not only the Arctic heat budget but also potentially the nutrient budget and the primary production.
Key Points
The decline of Arctic sea ice reduces its export, thus increasing the salinity in Greenland Sea
This reduces the sea surface height and speeds up the gyre circulation in Greenland and Nordic Seas
The consequently enhanced Atlantic Water transport intensifies the warming at Fram Strait and in the Arctic Ocean
Eddy‐resolving regional ocean model results in conjunction with synthetic float trajectories and observations provide new insights into the recirculation of the Atlantic Water (AW) in Fram Strait ...that significantly impacts the redistribution of oceanic heat between the Nordic Seas and the Arctic Ocean. The simulations confirm the existence of a cyclonic gyre around the Molloy Hole near 80°N, suggesting that most of the AW within the West Spitsbergen Current recirculates there, while colder AW recirculates in a westward mean flow south of 79°N that primarily relates to the eastern rim of the Greenland Sea Gyre. The fraction of waters recirculating in the northern branch roughly doubles during winter, coinciding with a seasonal increase of eddy activity along the Yermak Plateau slope that also facilitates subduction of AW beneath the ice edge in this area.
Key Points
Seasonally varying eddy‐mean flow interaction controls recirculation of Atlantic Water in Fram Strait
The bulk recirculation occurs in a cyclonic gyre around the Molloy Hole at 80 degrees north
A colder westward current south of 79 degrees north relates to the Greenland Sea Gyre, not removing Atlantic Water from the slope current
The long‐term dynamics of microbial communities across geographic, hydrographic, and biogeochemical gradients in the Arctic Ocean are largely unknown. To address this, we annually sampled polar, ...mixed, and Atlantic water masses of the Fram Strait (2015–2019; 5–100 m depth) to assess microbiome composition, substrate concentrations, and oceanographic parameters. Longitude and water depth were the major determinants (~30%) of microbial community variability. Bacterial alpha diversity was highest in lower‐photic polar waters. Community composition shifted from west to east, with the prevalence of, for example, Dadabacteriales and Thiotrichales in Arctic‐ and Atlantic‐influenced waters, respectively. Concentrations of dissolved organic carbon peaked in the western, compared to carbohydrates in the chlorophyll‐maximum of eastern Fram Strait. Interannual differences due to the time of sampling, which varied between early (June 2016/2018) and late (September 2019) phytoplankton bloom stages, illustrated that phytoplankton composition and resulting availability of labile substrates influence bacterial dynamics. We identified 10 species clusters with stable environmental correlations, representing signature populations of distinct ecosystem states. In context with published metagenomic evidence, our microbial‐biogeochemical inventory of a key Arctic region establishes a benchmark to assess ecosystem dynamics and the imprint of climate change.
Regional and vertical signatures of microbiology and biogeochemistry across Fram Strait. Western Strait, Arctic‐influenced: more dissolved organic carbon (DOC), fewer bacterial cells including Thioglobaceae and SAR406. Eastern Strait, Atlantic‐influenced: more carbohydrates (CHO), more bacterial cells including Thiotrichaceae and LS‐NOB. Inorganic nitrogen prevailed throughout the lower‐photic zone.
Two full‐year mooring records of sea‐ice, physical, and bio‐optical parameters illuminate tight temporal coupling between the retreating seasonal ice edge and the summer phytoplankton bloom on the ...Laptev Sea shelf. Our records showed no sign of pelagic under‐ice blooms despite available nutrients and thinning sea ice in early summer, presumably because stratification had not yet developed. Chlorophyll blooms were detected immediately after the ice retreated in late May 2014 and late July 2015. Despite radically different timing, the blooms were similar in both magnitude and length, interpreted as community‐level nutrient limitation. Acoustic backscatter records suggest the delayed 2015 bloom resulted in lower zooplankton abundance, perhaps due to a timing mismatch between ice algal and pelagic blooms and unfavorable thermal conditions. Our observations provide classical examples of ice‐edge blooms and further emphasize the complexity of high‐latitude shelves and the need to understand vertical mixing processes important for stratification and nutrient fluxes.
Key Points
Ice‐edge blooms were recorded with year‐round moorings in the Laptev Sea in 2014 and 2015
Timing of the bloom is variable and dependent on sea‐ice retreat
Lack of substantial under‐ice pelagic primary production presumably due to missing stratification
Submesoscale flows are energetic motions on scales of several kilometers that may lead to substantial vertical motions. Here we present satellite and ship radar as well as underway ...conductivity‐temperature‐depth and Acoustic Doppler Current Profiler observations of a cyclonic submesoscale filament in the marginal ice zone of Fram Strait. The filament created a 500‐m thin and 50‐km long sea ice streak and extends to >250‐m depth with a negative/positive density anomaly within/below the halocline. The frontal jets of 0.5 m/s are in turbulent thermal wind balance while the ageostrophic secondary circulation in places appears to subduct Atlantic Water at >50 m/day. Our study reveals the submesoscale dynamics related to sea ice shapes that can be sensed remotely and shows how submesoscale dynamics contribute to shaping the marginal ice zone. It also demonstrates the co‐occurrence and mixing of water masses over short horizontal scales, which has implications for ocean and sea ice models and understanding of patch formation of planktonic organisms.
Plain Language Summary
A sea ice streak in the marginal ice zone was observed with radar measurements. Below this streak in situ shipboard measurements of the temperature, salinity, and velocity field revealed a cyclonic submesoscale filament. This is a line of denser water of a few kilometers width bounded by strong counteracting velocities. This denser water is also associated with a different water mass and thus a change in biological properties and communities. This provides in situ confirmation for previous theoretical conclusions of how oceanic flows on kilometer scales structure the sea ice and biology in the marginal ice zone. The understanding of such small‐scale processes helps improve computer models of the ocean and sea ice dynamics. It also makes it possible to interpret oceanic flows from remote sensing of sea ice. Furthermore, it gives indication over which horizontal scales biological processes vary in the ocean.
Key Points
A submesoscale cyclonic filament in the marginal ice zone was observed with multiple techniques
The ageostrophic circulation accumulated sea ice and had local subduction of more than 50 m/day
Although previously unresolved, such filaments impact mixing, sea ice, and biological productivity
Abstract
The West Spitsbergen Current (WSC) is a topographically steered boundary current that transports warm Atlantic Water northward in Fram Strait. The 16 yr (1997–2012) current and ...temperature–salinity measurements from moorings in the WSC at 78°50′N reveal the dynamics of mesoscale variability in the WSC and the central Fram Strait. A strong seasonality of the fluctuations and the proposed driving mechanisms is described. In winter, water is advected in the WSC that has been subjected to strong atmospheric cooling in the Nordic Seas, and as a result the stratification in the top 250 m is weak. The current is also stronger than in summer and has a greater vertical shear. This results in an
e
-folding growth period for baroclinic instabilities of about half a day in winter, indicating that the current has the ability to rapidly grow unstable and form eddies. In summer, the WSC is significantly less unstable with an
e
-folding growth period of 2 days. Observations of the eddy kinetic energy (EKE) show a peak in the boundary current in January–February when it is most unstable. Eddies are then likely advected westward, and the EKE peak is observed 1–2 months later in the central Fram Strait. Conversely, the EKE in the WSC as well as in the central Fram Strait is reduced by a factor of more than 3 in late summer. Parameterizations for the expected EKE resulting from baroclinic instability can account for the observed EKE values. Hence, mesoscale instability can generate the observed variability, and high-frequency wind forcing is not required to explain the observed EKE.
The hydrography of the Arctic Seas is being altered by ongoing climate change, with knock‐on effects to nutrient dynamics and primary production. As the major pathway of exchange between the Arctic ...and the Atlantic, the Fram Strait hosts two distinct water masses in the upper water column, northward flowing warm and saline Atlantic Waters in the east, and southward flowing cold and fresh Polar Surface Water in the west. Here, we assess how physical processes control nutrient dynamics in the Fram Strait using nitrogen isotope data collected during 2016 and 2018. In Atlantic Waters, a weakly stratified water column and a shallow nitracline reduce nitrogen limitation. To the west, in Polar Surface Water, nitrogen limitation is greater because stronger stratification inhibits nutrient resupply from deeper water and lateral nitrate supply from central Arctic waters is low. A historical hindcast simulation of ocean biogeochemistry from 1970 to 2019 corroborates these findings and highlights a strong link between nitrate supply to Atlantic Waters and the depth of winter mixing, which shoaled during the simulation in response to a local reduction in sea‐ice formation. Overall, we find that while the eastern Fram Strait currently experiences seasonal nutrient replenishment and high primary production, the loss of winter sea ice and continued atmospheric warming has the potential to inhibit deep winter mixing and limit primary production in the future.
Plain Language Summary
The Fram Strait is the main gateway of the Arctic Ocean. In the east, warm, salty waters from the Atlantic flow north into the Arctic basin, and in the west, cold, fresh waters flow south from the central Arctic into the North Atlantic. We examined how changes to the availability of nutrients (which are essential for algae to grow) may limit algae growth in the Fram Strait, both as a result of changes to their source and also how easily the upper ocean mixes nutrients from depth. In the eastern Fram Strait, there is a high availability of nitrate, one of the main nutrients to support algae growth, and winter mixing sustains nutrient supply and biological production in recent decades. However, in the western Fram Strait, the outflowing surface waters do not easily mix with deeper waters and are depleted in nitrate, and nutrient supply from the central Arctic has been declining in recent decades. Our work suggests that although the eastern Fram Strait is sustaining higher levels of algae growth, which supports fisheries and higher trophic levels, warming over the coming decades could shoal winter mixed layers enough to decrease summertime nutrients and limit biological production.
Key Points
Nitrate isotope signatures in the western Arctic outflow of Polar Surface Water and eastern inflow of Atlantic Water are characterized
Western Fram Strait is strongly stratified and nitrate deplete compared to the east where winter mixing sustains nutrient supply
Future warming may shoal the winter mixed layer in the east, decreasing nitrate supply, and reducing primary production below current rates
The spatial distribution of 29 per- and polyfluoroalkyl substances (PFASs) in seawater was investigated along a sampling transect from Europe to the Arctic and two transects within Fram Strait, ...located between Greenland and Svalbard, in the summer of 2018. Hexafluoropropylene oxide-dimer acid (HFPO-DA), a replacement compound for perfluorooctanoic acid (PFOA), was detected in Arctic seawater for the first time. This provides evidence for its long-range transport to remote areas. The total PFAS concentration was significantly enriched in the cold, low-salinity surface water exiting the Arctic compared to warmer, higher-salinity water from the North Atlantic entering the Arctic (260 ± 20 pg/L versus 190 ± 10 pg/L). The higher ratio of perfluoroheptanoic acid (PFHpA) to perfluorononanoic acid (PFNA) in outflowing water from the Arctic suggests a higher contribution of atmospheric sources compared to ocean circulation. An east-west cross section of the Fram Strait, which included seven depth profiles, revealed higher PFAS concentrations in the surface water layer than in intermediate waters and a negligible intrusion into deep waters (>1000 m). Mass transport estimates indicated a net inflow of PFASs with ≥8 perfluorinated carbons via the boundary currents and a net outflow of shorter-chain homologues. We hypothesize that this reflects higher contributions from atmospheric sources to the Arctic outflow and a higher retention of the long-chain compounds in melting snow and ice.
Abstract
Data from a closely spaced array of moorings situated across the Beaufort Sea shelfbreak at 152°W are used to study the Western Arctic Shelfbreak Current, with emphasis on its configuration ...during the summer season. Two dynamically distinct states of the current are revealed in the absence of wind, with each lasting approximately one month. The first is a surface-intensified shelfbreak jet transporting warm and buoyant Alaskan Coastal Water in late summer. This is the eastward continuation of the Alaskan Coastal Current. It is both baroclinically and barotropically unstable and hence capable of forming the surface-intensified warm-core eddies observed in the southern Beaufort Sea. The second configuration, present during early summer, is a bottom-intensified shelfbreak current advecting weakly stratified Chukchi Summer Water. It is baroclinically unstable and likely forms the middepth warm-core eddies present in the interior basin. The mesoscale instabilities extract energy from the mean flow such that the surface-intensified jet should spin down over an e-folding distance of 300 km beyond the array site, whereas the bottom-intensified configuration should decay within 150 km. This implies that Pacific Summer Water does not extend far into the Canadian Beaufort Sea as a well-defined shelfbreak current. In contrast, the Pacific Winter Water configuration of the shelfbreak jet is estimated to decay over a much greater distance of approximately 1400 km, implying that it should reach the first entrance to the Canadian Arctic Archipelago.
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